Vertebrates
A chordate is an constituting the Chordata. During some period of their life cycle, chordates possess a , a , s, an , and a post-anal : these five anatomical features define this phylum. Chordates are also ; and have a , , and a . The Chordata and together form the . Chordates are divided into three : ( , s, s, s, and s); ( s and s); and (which includes s). There are also extinct taxa such as the . (which includes the s) has been presented as a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and ata form the , the sister phylum of the Chordates. Of the more than 65,000 living species of chordates, about half are bony fish that are members of the . Chordate s have been found from as early as the , 541 million years ago. ( ), s – chordates with the notochord replaced by a during development – are considered to be a subgroup of the , which consists of chordates with a . The Craniata and Tunicata compose the clade . (See diagram under Phylogeny.) Vertebrates comprise all species of s within the Vertebrata ( with ). Vertebrates represent the overwhelming majority of the , with currently about 69,963 described. Vertebrates include such groups as the following: * * , which include the ( s, , and ratfish) * , which include s, s, s and s * vertebrates range in size from the species , at as little as , to the , at up to . Vertebrates make up less than five percent of all described ; the rest are , which lack s. The vertebrates traditionally include the , which do not have proper e due to their loss in evolution, though their closest living relatives, the s, do. Hagfish do, however, possess a . For this reason, the vertebrate subphylum is sometimes referred to as " " when discussing morphology. Molecular analysis since 1992 has suggested that hagfish are most closely related to lampreys, and so also are vertebrates in a sense. Others consider them a sister group of vertebrates in the common taxon of craniata. Phylogenetic tree |sublabel1= |sublabel1= |1= |sublabel1= |1= (hagfish) |2= (Petromyzontida) (Lampreys) }} }} |2= † |2= † |3= a† |4= † |5= † |6= † |7= |sublabel1= |1= † |2= † |2= † |2= † |label3=Crown |sublabel3=Gnathostomata |3= , incl |label2= |2= (ray-finned fish) |2= |sublabel2=(lobe‑finned fish) |1= (coelacanths) |2= (lungfishes) |2= a |1= |label2= |2= ia |2= }} }} }} }} }} }} }} }} }} }} }} }} }} }} }} ''' |1= |sublabel1= |1= (lampreys) |2= }} |2=?† |label3=unnamed |3= |2=?† |label3=unnamed |3= |label2=unnamed |2= |label2=unnamed |2= |2=† |label3= |3= (armoured fishes) |label2= unnamed |2= , incl. (cartilaginous fishes) |label2= |2= (ray-finned fishes) |label2= |sublabel2=(lobe-finned fish) |2= |2= (coelacanths) |label3= unnamed |3= |2= (lungfishes) }} |label4= unnamed |4= |2= |2= }} }} }} }} }} }} }} }} }} }} }} }} References Category:Tree of life